In Situ Spectroscopic Ellipsometry for the Real Time Process Control of Plasma Etching of Silicon Nitride
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sample property or properties of interest are the fit parameters. The strength of ellipsometry as an in-situ sensor for etching control lies in its sensitivity to film thickness and the nondestructive, non-invasive nature of SE measurements. In the present work, we describe the design, testing and evaluation of an SE based real time feedback controller for etch rate regulation in CF 4 /0 2 plasma etching of silicon nitride films. An adaptive, variable gain controller is designed, tested, and evaluated. The feedback variable is the current etch rate as determined from the in-situ SE measurements of the film's thickness. The controller compensates for drifts in etch rate which occur during a given etch, and adaptively adjusts for the run-to-run variability inherent to plasma processing. An outline of the remainder of the paper is as follows. In the next section we discuss the experimental setup. In the third section we describe spectroscopic ellipsometry, and in the fourth section we describe the design and development of our adaptive feedback controller. In a final fifth section, our experimental results are presented and discussed. EXPERIMENTAL SETUP The etch chamber is equipped with an electron cyclotron resonance source (Astex). A spectroscopic ellipsometer (J. A. Woollam Co., type M-88) is mounted on the chamber to provide real-time sensing of material properties such as film thickness and surface roughness. This will be described in greater detail below. The working pressure is measured by a 263 Mat. Res. Soc. Symp. Proc. Vol. 5910 2000 Materials Research Society
manometer (MKS type 690A). The inflow rate of CF 4 / 5% 02 gas (pre-mixed) is controlled by a mass flow controller to achieve a constant inflow rate of 10 sccm. The chamber pressure is established by means of the throttle valve mounted on the pumping port and, before the microwave power is applied, is approximately 5.85 mTorr. After the plasma is present, the chamber pressure increases due to dissociation of the gas, interaction with the chamber walls and possibly even heating effects of the plasma, saturating at a value of approximately 6.40 mTorr. The entire ECR etching system is monitored and controlled by a 486 CPU PC equipped with a National Instruments AT-MIO-64E-3 A/D card, AT-AO-10 D/A card, PC-DIO-96 digital I/O card, and 4-port RS-232 card.
SPECTROSCOPIC ELLIPSOMETRY Ellipsometry is a widely used technique for the nondestructive measurement of bulk and multilayer structures of such materials as metals, dielectrics, semiconductors, and polymers
(see, for example, [2]). In ellipsometry, polarized light strikes the sample surface and the polarization of the reflected light is analyzed by a second polarizer. The measured change in polarization (p) is commonly expressed in terms of the complex Fresnel reflection coefficients for the components of the incident and reflected light in the p-plane (the plane parallel to the plane of incidence) and in the s-plane (the plane perpendicular to the plane of incidence), Fp, S=-
EorE01/E. E=/EV'
(1)
w
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